Can rockets be recycled? Check out these mind-blowing tricks!

Recently, Rocket Lab, an American company, revealed that due to repeated failures in helicopters to grab and recover rockets in the air, it is considering changing the plan to recover splashed rocket sub-stages from the sea for refurbishment and reuse. This means that the increasingly popular rocket recovery business has ushered in a "change of track". So, what are the advantages and disadvantages of the several existing rocket recovery methods in the aerospace industry? What kind of brain-opening rocket recovery tricks can people expect to see in the future?

Helicopters have repeatedly failed to grab and recover rockets in the air (Source: Rocket Lab)

Traditional recycling model has challenges

Although more and more spacecraft are flying into space, researchers are never satisfied with one aspect: how to help spacecraft "break free from the embrace of Mother Earth" more safely and economically? In professional terms, how to reduce the overall cost of space launch missions? Rocket recovery is undoubtedly one of the most direct means.

Judging from the current status of rocket recovery operations, the Falcon series of rockets of the US SpaceX company has the most mature technical model, using a vertical self-return solution, and has successfully recovered more than 100 times. The first stage of multiple Falcon 9 rockets has repeatedly completed 15 launch missions.

However, this method of rocket recovery is also inevitable. It is necessary to reserve at least 1/3 of the fuel reserve for the first stage of the rocket so that the secondary ignition can be carried out in time to slow down. In other words, in order to achieve reuse, the rocket has to lose its carrying capacity. In addition, in order to ensure that the first stage of the rocket lands smoothly in a vertical upright position, buffer legs are indispensable. Only medium and large rockets can "tolerate" this weight and space cost, which invisibly limits the scope of application.

Therefore, Rocket Lab proposed a helicopter aerial grabbing and recovery rocket plan in 2021. Simply put, it is to let the lightweight Electron rocket stage descend steadily under the parachute, and the helicopter approaches in time, extends the probe hook, and hangs and recovers it. Although outsiders feel that this recovery method is both exciting and efficient, the response ability, lifting capacity, and reliability of the "receiving" helicopter are facing stringent requirements. In order to ensure the "date timing", it is also difficult to achieve accurate data transmission control.

In one Electron rocket recovery mission, the helicopter's cable device had been successfully suspended, but the sudden huge load in the air almost caused the helicopter to lose control, forcing the helicopter to regretfully "let go". In another recovery mission, due to the loss of telemetry data when the rocket substage returned to the atmosphere, the helicopter on standby could not locate it in time and unfortunately missed the "date".

In addition, countries have also tested recovery modes such as multiple parachutes and reverse rockets, but none of them are suitable for recovering rocket compartments, and more technical exploration is still needed. As for recovering rocket sub-stages that splash down at sea, the requirements for materials and timing are undoubtedly more stringent.

There are many routes to "pass the customs"

Because rocket recovery is not easy, once aerospace professionals master a mature solution, they can expect to reap rich rewards. Therefore, from aerospace enthusiasts to senior scientists, a series of dazzling new rocket recovery solutions are constantly proposed, and multiple routes are advancing in parallel, striving to find a "pass secret" with higher level and greater benefits.

In recent years, aerospace personnel from many countries have proposed the idea of ​​using rope nets to suspend and recover rockets, which has attracted considerable attention. The common point of the plans of various countries is to build a rectangular recovery platform, with the four corners stabilized by towering towers, connecting many cables, arranged in multiple layers, and each layer is in the shape of a "well", forming a strong and tough large network with complex forces. It is expected that when the rocket sub-stage falls into the sensing area in a nearly vertical posture, multiple sets of parallel cables in the first layer of cables will move quickly with the help of sensors, accurately align with the falling direction of the rocket sub-stage, and hang on components such as the tail hook, thereby triggering the force on all cables in the first layer. As the rocket sub-stage continues to decelerate and fall, each layer of cables is suspended in the same way until the rocket sub-stage stops falling and is safely recovered.

To ensure the reliability of the cable hooking the rocket sub-stage, some plans add clamps made of special materials, which can make the rocket subject to relatively slight lateral force, and the deceleration effect may be more significant.

Obviously, this novel recovery method does not require the rocket to reserve a large amount of fuel, and can also save components such as buffer legs, which is expected to significantly improve the carrying capacity and greatly reduce the difficulty of controlling the rocket to maintain its attitude. However, in order to "intercept" the rocket sub-stage in time, the cable material and the entire system mechanism will face a tough test in terms of strength and accuracy. In addition, the precise control of the rocket sub-stage ejection and weather conditions may affect the success or failure of its "self-entrapment".

Perhaps because it is too difficult and troublesome to find the landing point of the rocket substage during its descent, space enthusiasts have proposed a simpler and more brutal solution in recent years - tiankeng recycling. Simply put, it is to use natural geological structures, or dig a large tiankeng with a depth of tens of meters and a diameter of more than 10 kilometers, and fill it with a large amount of cushioning materials, which can be recycled waste plastics or various types of plant fibers.

The rocket recovery process is also very simple. As long as the landing point is within the range of the tiankeng, the tens of meters thick cushioning material can largely ensure the safety of the mission. Next, the helicopter on standby nearby quickly flew over the tiankeng, lowered the hook, lifted the rocket sub-stage, and transported it back to the workshop to prepare for the next launch mission.

The tiankeng recycling model sounds a bit ridiculous, but it can reduce the material cost and technical difficulty of the rocket substage on its way back, and there are not many technical obstacles to establishing an alternative recycling site. The problem is that the construction of a huge tiankeng must be very expensive, and the open-air recycling site is exposed to wind, rain, and scorching sun. What is the state of the buffer material? How to maintain it in time? I am afraid the proposer has no idea.

In fact, among the many fantastic ideas for rocket recovery, there is one plan that is only in the conceptual stage, but is quite perfect in theory, that is self-gliding return. Simply put, it is to equip the rocket sub-stage with foldable wings and servos, so that it can "transform" in time to ensure safety.

As we all know, the ejection point of a rocket substage is often tens of thousands of meters in the air, with huge potential energy. As the rocket substage is in free fall, the potential energy is converted into huge kinetic energy, causing it to continue to accelerate down. The conventional method of protecting the rocket substage is to try to offset this extremely destructive kinetic energy in the vertical direction and slow down through "soft and hard" means. The self-gliding return plan is to "turn danger into opportunity" and allow the rocket substage to fly using kinetic energy.

During the accelerated fall of the rocket sub-stage, if the wings and servos are deployed in time, with the support of unmanned driving technology, it is expected that the rocket sub-stage will be able to quickly change from a free fall state to a horizontal flight state, like a glider diving from a high altitude, but it pursues a more stable flight, continuously consumes terrible kinetic energy, and finally lands safely.

In the future, when the rocket's appearance design is more optimized, and the materials, processes and unmanned intelligent driving technology reach a higher level, then the rocket sub-stage will autonomously search, locate and fly back to the launch base, and land smoothly on the runway, completing the "gorgeous transformation" from a spacecraft to an aircraft. This is not just a pipe dream. (Author: Sun Fei)